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Depends on the type of engine.
One gallon of gasoline weighs 5 to 6 pounds depending on temperature. 2,000 pounds divided by 6 pounds per gallon gives 333 gallons.
It depends on the formulation of the gasoline (for example how much ethanol is added), but it's about 114,000 BTU {see GGE table in related Wikipedia link}. GGE is a National Institute of Standards...
42 gallons per barrel.
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